1. Field of the Invention
The present invention relates to a projection optical apparatus. More particularly, the invention relates to a projection optical apparatus suitably applicable as such an apparatus as a film projector and a microfilm reader printer, which may prevent light amount unevenness on a surface of a screen, due to a luminous intensity distribution (luminous intensity property) of a light source, by properly setting an NA (numerical aperture) of a projection lens on a screen side depending upon a projection magnification when an image to be projected, for example an image on a slide, is enlarged for projection onto the screen surface.
2. Related Background Art.
In conventional projection optical apparatuses such as a film projector and a microfilm reader printer, an optical flux from light source means is converged by using a condenser lens and a field lens to illuminate an image to be projected (projection image) of a slide or of a liquid crystal display device (transmission type). Then the projection image is enlarged and projected by a projection lens at a predetermined projection magnification onto a surface of a screen.
FIG. 1 is a main part schematic drawing of an optical system in a conventional projection optical apparatus.
In FIG. 1, an optical flux radiated from light source means 51 comprising a halogen lamp passes through a condenser lens 52, a heat absorbing filter 53, and a field lens 54 to illuminate an effective illumination area of a projection image 55 of transmission type on a slide or the like located at a plane 55a to be illuminated. A projection lens 56 enlarges to project the projection image 55 thus illuminated onto a surface of a screen 60 at a predetermined magnification.
In the projection optical apparatus as shown in FIG. 1, the optical flux from the light source means 51 is converged (imaged) at or near an entrance pupil 58 of the projection lens 56. That is, the optical elements are arranged to image a lighting plane (filament) 57 of the light source means 51 at or near the entrance pupil 58.
Further, an NA (numerical aperture) on the side of the screen 60 is arranged as unchanged with a change in projection magnification to maintain brightness of total projection image on the surface of the screen 60 unchanged in various changes of projection magnification.
There is such a problem recognized in the projection optical apparatus as described that unevenness in light amount is caused by a luminous intensity distribution of the light source means 51 at a low projection magnification on the surface of the screen 60 if a ratio of projection magnifications, or magnification change ratio exceeds a certain level.
The reason why the light amount unevenness occurs is as follows.
A halogen lamp is generally used as the light source means in conventional projection optical apparatuses. The halogen lamp tends to have unevenness in luminous intensity property due to a structure of its lighting plane (filament).
FIGS. 2A and 2B are explanatory drawings to show a perspective view and a luminous intensity property of a general halogen lamp. The x-, the y-, and the z-axes are defined as shown in FIG. 2A.
Since there is unevenness in luminous intensity property in the xy section of the halogen lamp as shown in FIG. 2B, there are directions of bright optical flux and directions of dark optical flux depending upon a radiation direction as shown in FIG. 3 when the halogen lamp is seen from above.
FIG. 4 is a main part schematic drawing of an optical system in a conventional projection optical apparatus to show occurrence of light amount unevenness on a screen surface when the halogen lamp having such a luminous intensity property is used as the light source means.
As shown in FIG. 4, among optical fluxes radiated from the light source means 51, an optical flux passing through an illumination optical system 50 and then near a central portion 55A of a projection image 55 is bright in luminous intensity property of the light source means 51. When the projection image 55A illuminated by this optical flux is enlarged and projected onto a surface of the screen 60 by the projection lens 56, a central portion of the surface of the screen 60 becomes bright in a projected image.
In contrast, among the optical fluxes radiated from the light source means 51, an optical flux passing through the illumination optical system 50 and then through a peripheral portion 55B of the projection image 55 is dark in luminous intensity property of the light source means 51. When the projection image 55B illuminated by this optical flux is enlarged and projected onto the surface of the screen 60 by the projection lens 56, a peripheral portion of the screen surface becomes dark in the projected image.
For the above reason, the light amount unevenness occurs from the luminous intensity distribution of the light source means 51, for example by which the central portion is bright and the peripheral portion is dark on the surface of the screen 60.
The above-described light amount unevenness on the surface of the screen 60, however, may be made ambiguous by properly selecting an imaging magnification, at which a lighting plane (filament) 57 is imaged by the condenser lens 52 and the field lens 54 at near the entrance pupil 58 of the projection lens 56, and an F.sub.NO (f-number) of the projection lens 56.
Namely, it may be accomplished by setting larger an angular aperture or NA (numerical aperture) of optical fluxes radiated from the light source means 51 and then incident into tile condenser lens 52.
FIGS. 5A and 5B are explanatory drawings of the light source means 51 and neighbors when the light amount unevenness is corrected by increasing the NA of the incident optical fluxes from the light source means 51 into the condenser lens 52.
As seen in FIGS. 5A and 5B, either optical fluxes advancing in A direction (for example toward tile central portion of the screen surface) or optical fluxes advancing in B direction (for example toward the peripheral portion of the screen surface) include optical fluxes both of bright portion and of dark portion in the luminous intensity distribution of the light source means 51, consequently averaging the luminous intensity distribution.
According to the above arrangement, a difference in brightness may be nullified between the optical fluxes advancing in A direction and in B direction, so that the occurrence of light amount unevenness caused by the luminous intensity distribution of the light source means may be minimized.
An increase in NA of the incident optical fluxes from the light source means 51 into the condenser lens 52 may be accomplished by decreasing the F.sub.NO of the projection lens 56, or by increasing the imaging magnification at which the lighting plane (filament) 57 is imaged at or near the entrance pupil 58 of the projection lens 56.
In a general projection optical apparatus, such as a film projector and a microfilm reader printer, which has a fixed projection magnification of the projection lens, or, in one which has a variable projection magnification in a magnification change ratio, which is a ratio of projection magnifications, of 3 or less, the occurrence of the light amount unevenness may be effectively prevented on the screen surface in the state as shown in FIGS. 5A, 5B for the incident optical fluxes from the light source means into the condenser lens, that is, by making the NA thereof large.
However, once the magnification change ratio of the projection optical apparatus exceeds three, it becomes difficult to always maintain large the NA of the incident optical fluxes from the light source means into the condenser lens, even by using a projection lens with any projection magnification. It is generally preferable that the illuminance is always constant on the screen surface with a change in projection magnification of the projection lens in a projection optical apparatus such as a film projector and a microfilm reader printer. Therefore, an NA on the screen side is conventionally set to always become constant irrespective of the change in projection magnification of the projection lens.
However, if the NA of the projection lens on the screen side is made constant, the NA of the projection lens on the projection image side changes with a change in projection magnification. For example, letting a projection magnification of the projection lens be M, an NA on the screen side be NA.sub.S, and an NA on the projection image side be NA.sub.O, there is a relation among them as follows: EQU NA.sub.O =M.times.NA.sub.S.
If the NA of the projection lens on the screen side (as will be referred to as NA.sub.S) is fixed, the NA of the projection lens on the projection image side (as will be referred to as NA.sub.O) is proportional to the projection magnification M. That is, NA.sub.O is proportional to the projection magnification M of the projection lens.
Further, since the NA of the incident optical fluxes from the light source means into the condenser lens (as will be referred to as NA.sub.L) is proportional to NA.sub.O, the NA.sub.L changes in proportion to the projection magnification M of the projection lens after all.
The NA.sub.L will be compared in an example in which two projection lenses different in projection magnification M are used, one with a low magnification of 10 times and the other with a high magnification of 50 times, presenting a magnification change ratio of 5.
For example, suppose that a value of the NA.sub.L is 0.5 with a projection magnification of 50 times. Then a value of the NA.sub.L becomes 0.1 with a projection magnification of 10 times. If the NA.sub.L value should be set over 0.8 (angular aperture of about 110 degrees), a condensing efficiency could not be improved in actual in view of the property of the luminous intensity distribution of halogen lamp as shown in FIG. 2B. Also, it becomes difficult to maintain performance of imaging of the illumination optical system. It is, therefore, generally preferable that the NA.sub.L value is set to be about 0.8 at most.
For example, if the NA.sub.L value is set to be 0.8 with the projection magnification of the projection lens being 50 times, the NA.sub.L value is only 0.16 with the projection magnification of the projection lens being 10 times. The NA.sub.L value of 0.16 corresponds to an angular aperture of 9.2 degrees.
Thus, a state of projection for the latter is the same as the projection state as shown in FIG. 4. The light amount unevenness is eventually caused by the luminous intensity distribution of the light source means at a low magnification on the screen surface.
As explained, in case that the magnification change ratio becomes greater to some extent in the conventional projection optical apparatuses, there is a problem of occurrence of light amount unevenness caused by the luminous intensity distribution of the light source means at a low magnification on the screen surface.
Although it is supposed in the above explanation that the magnification change ratio of the projection optical apparatus is 5, the light amount unevenness is likely to occur with a magnification change ratio of about 3.